In an electrophotographic image forming apparatus, a charging device charges an outer circumferential surface of an image carrying member via a charging member such as a charging roller. In addition, a laser scanning unit writes an electrostatic latent image on the outer circumferential surface of the image carrying member. Furthermore, a developing device develops the electrostatic latent image to a toner image.
Charging the outer circumferential surface of the image carrying member to a predetermined target potential is important to obtain a toner image of an excellent image quality. Accordingly, a charging voltage applied to the charging member is controlled so that the outer circumferential surface of the image carrying member is charged to the target potential. In that case, a surface potential sensor that detects the surface potential of the image carrying member in a contactless manner is used.
In addition, it is known that a controller controls the charging voltage based on detected values of the temperature and humidity of the installation environment, and a current flowing between the charging member and the image carrying member (see, for example, PTL 1). The detected values of the temperature, humidity, and current are indirect parameters that affect the surface potential of the image carrying member.